The present invention relates to a diaphragm electrolysis of an alkali metal chloride.
The conventional asbestos diaphragm used for a diaphragm electrolysis of an alkali metal chloride is classified into three types; a diaphragm using the asbestos fiber without any prefabrication, a diaphragm of a cloth obtained by weaving the asbestos fiber, and a diaphragm of a sheet obtained by sheeting the asbestos fiber.
When the asbestos fiber is used without any prefabrication, the asbestos fiber is suspended in an solution of a base such as sodium hydroxide and a wire net cathode is dipped in the suspension and the solution is removed under a suction so as to deposit the asbestos fiber on the wire whereby the diaphragm is obtained.
When the diaphragm is prepared by such a method, the commercial asbestos fiber is packed under compression for the purpose of reducing the bulk from the viewpoint of transportation efficiency. Accordingly, the asbestos fiber is fluffed when used for the preparation of the diaphragm.
The operation for fluffing the asbestos fiber is preferably performed while minimizing the break of asbestos fiber and accordingly, a hand operation for fluffing the asbestos fiber has been employed. The hand operation for fluffing the asbestos fiber requires great labour and a long time and it is subjected to regulation for environmental hygiene to prevent the scattering of the asbestos fiber in the atmosphere.
In an electrolysis of an alkali metal chloride by using an asbestos diaphragm prepared by using the asbestos fiber fluffed by the hand operation, the concentration of sodium hydroxide in a catholyte is different among the individual cells. For example, in a bipolar type electrolytic cell having eleven electrolytic compartments (elements), the concentration of sodium hydroxide is distributed in a wide range from 155 to 135 g/liter. This causes a local deterioration which affects the life of the diaphragm and this disadvantageously affects the operating life of the electrolytic cell.
Accordingly, it is necessary to decrease the concentration difference of sodium hydroxide among the compartments and to increase the concentration of sodium hydroxide in the catholyte and thusly reduce the amount of steam used in the concentrating process for the sodium hydroxide.
The conventional asbestos diaphragm also has disadvantages of lower current efficiency and high concentration of sodium chlorate in the catholyte.